tinothrips rufus (Gmelin)-were able to survive as adults in the vegetation sampled, but only Anaphothripini survived as larvae. T. tabaci adults overwinter within ...
Overwintering of the Onion Thrips, Thrips tabaci (Thysanoptera: Thripidae), in New York R. C. NORTH
AND
A. M. SHELTON
Department of Entomology, New York State Agricultural Experiment Station, Cornell University, Geneva, New York 14456
Environ. Entomol. 15: 695-699 (1986)
A survey of overwintering thrips and the potential for Thrips tabaci Lindeman to overwinter in upstate New York were investigated during the winters of 1982-83 and 1983-84. Six species-To tabaci, Anaphothrips obscurus (Miiller), Chirothrips manicatus Haliday, Limothrips denticornis (Haliday), Frankltniella tenuicornis (Uzel), and Aptinothrips rufus (Gmelin)-were able to survive as adults in the vegetation sampled, but only Anaphothripini survived as larvae. T. tabaci adults overwinter within winter wheat, alfalfa, and weedy vegetation, and these overwintering females are capable of ovipositing on these plants during the spring. Winter wheat, oats, and alfalfa were the best oviposition and reproduction hosts we examined. ABSTRACT
A PREVIOUS STUDY BY Shelton & North (1986) focused On determining the possible origins of thrips entering and infesting cabbage fields in upstate New York. Although that study examined the species composition and phenology of all Thysanoptera on the various field and forage crops examined, particular emphasis was placed On Thrips tabaci Lindeman, an important pest of cabbage and onions in the state. Before the appearance of thrips in cabbage and onion fields, significant numbers of adult females are present in various field and forage crops in early to midsummer. The origin of the founders of the thrips within the cereal and forage crops is unknown. They could be long-distance migrants that arrive in New York in the spring, as has been suggested for the movement of some thrips into northern latitudes (Stannard 1968), or they could overwinter here. Shirck (1951) reported collecting T. tabaci adults from alfalfa and red clover in Idaho from September to May, and Boyce & Miller (1953) collected a mean of < 1 adult per sample from alfalfa, red clover, winter wheat, oat, and grass sod during the coldest time periods (January-February) in southern Ontario, Canada. The present study was initiated in 1982 to determine 1) the species composition of thrips that overwinter in several field crops and surrounding vegetation; 2) the ability of T. tabaci to overwinter in winter wheat and alfalfa and the ability of the survivors to reproduce on onions; and 3) the suitability of several hosts for T. tabaci oviposition and larval development. Materials and Methods Survey of Overwintering Thrips. Vegetative samples were examined for overwintering thrips from November through mid-April of 1982-83 and 1983-84. The samples consisted of four 320-cm2 695
subsamples, with three samples per field per date. The four subsamples consisted of the top 2 cm of soil in which vegetation was growing. All four subsamples were collected within a 2-m radius and placed together in labeled plastic bags. Thrips were extracted into a 70% ethanol and 1% formaldehyde solution in Berlese funnels (60-W bulb) for 3 days, and subsequently enumerated and identified. The larvae were identified to the tribe level utilizing the classification of Vance (1974). In 198283, samples were taken at 2-week intervals in three winter wheat, three alfalfa, two volunteer oat, and three cabbage fields, and six ditch areas adjacent to harvested cabbage fields. During 1983-84, two barley, three winter wheat, and three alfalfa fields, and six ditch areas were sampled (cabbage and oats were discontinued because no thrips were found throughout the previous year's sampling). T. tabaci Overwintering on Wheat and Alfalfa. T. tabaci adults (1982-83) and adults and larvae (1983-84) were placed On winter wheat and alfalfa plants contained in plastic pots covered with screening (Nytex 85 ILm, Tetko, Elmsford, N.Y.). This procedure was conducted in three winter wheat and three alfalfa fields in both winters. A cylinder was used to remove a plug (10 cm diam, 10 cm deep) of winter wheat or alfalfa and their surrounding soil from commercial fields. Twelve plugs were collected from each of three rows in the alfalfa and winter wheat fields in October 1982-83 and 1983-84. The soil and foliage was placed in a plastic pot (10 cm diam). The bottom of the plug was trimmed so that the wheat shoots and alfalfa stems were not crushed or matted down when Nytex screen was placed over the pot. Within 48 h of collection of plugs, T. tabaci adults were individually removed from heads of cabbage and placed onto the wheat and alfalfa. After 25 adults (1982-83) were placed onto the foliage in each pot, they were covered with N ytex screen. The
696
ENVIRONMENTAL
Vol. 15, no. 3
ENTOMOLOGY
Table 1. Species of thrips collected from different vegetation during the winters of 1982-84; to tribe only 1982-83
1982-84 Wheat
Alfalfa
Weedy vegetation
Oats
larvse are identified
1983-84 Cabbage
Barley
Nov. T. tabacl
A. omcuf'tlS C. mantcatus" F. tnucf" F. tenufcornfs Thripini Anaphothripini
T. A. F. F.
tabact omcurus tnucta tenutcornfs"
T. tabact A.omcuf'tlS C. mantcatus F. tenutcornfs A. rufus
Thripinia Anaphothripini
Thripinib Anaphothripini
T. tabacl A.omcuf'tlS F. tenufcorntsh
T. tabacl A. obscurus F. tenutcornfs
T. tabact A. obscurus F. tenutcorntsh A. rufus
Thripini Anaphothripini
Anaphothripini
Anaphothripini
T. tabact A.obscuf'tlS
T. tabact A. omcuf'tlS
Thripini Anaphothripini
F. tenutcornfs"
T. tabact A.omcuf'tlS A. rufus
Anaphothripini
Anaphothripini
T. tabact
T. tabacl A. obscurus Anaphothripini
T. tabact
1'. tabact
A. omcurus Thripini
L. clenUcomfs Thripini
T. tabacl A. obscuru.s
T. tabact
T tabact
Thripini
L. denticornfs T;uipini
Dec.
Jan. T. tabacf A. omcuru.s F. tenutcornfs"
T. tabacl
A.obscuf'tlS
Thripini
L. dentfcomfs
T. tabact
L. denUcomts
Feb. T. tabact A.omcuf'tlS F. tenufcornfs"
A. omcurus
T. tabact A.omcuf'tlS A. rufus
Anaphothripini
Anaphothripinia
Anaphothripini
T. tabact A. obscuf'tlS
T. tabact A.obscuf'tlS C. mantcatus" F. tenuicornfs" A. rufus
Anaphothripinia
Anaphothripini
T. tabact A.obscuf'tlS F. tnucta
T. tabact A.omcuf'tlS F. tnucta F. tenuicornfs A. rufus
Mar. T. tabact A.omcuf'tlS F. tenutcornw>
T. tabact
L. denticomts
Apr. T. tabact A.obscuf'tlS F. tnucta F. tenuicornfs
L. denticornts
Anaphothripini Chirothripinia a 1982-83 only. b 1983-84 only.
pots containing the foliage and soil and thrips were returned to the same fields from which they were removed and set into holes so that the top of the soil in the pots was even with the soil surface of the field. Each pot per row was randomly assigned a number from 1 to 12, and this number determined the week that it was to be returned to the laboratory and examined. A set of samples, three for each field, was returned every other week beginning 16 November 1982 and ending 19 April 1983. The same procedures were followed during the winter of 1983-84 except that 50% of the pots per row received 25 larvae rather than adult T. tabacl, and samples were examined at 3-week intervals from 13 December 1983 to 27 March 1984. The pots were returned to the laboratory and stored at 3-4°C until they could be examined (within 48
h of retrieval). Individual pots were removed from the cooler and taken to the laboratory: where the Nytex covering was carefully removed. Individual wheat shoots or alfalfa stems were ca:~efully severed at the base and placed on a white surface. Individual shoots or stems were examined as they were taken apart, and both living and dead thrips were counted. The soil and plant root:; were also examined. The top 2-3 cm of soil was carefully removed and placed on a white surface and examined microscopically for thrips. The remaining soil was placed into a saturated NaCI solution and gently agitated. After a 10-min period to allow the thrips to float, the solution was poured through Nytex screening and the thrips were counted and identified. Live T. tabaci adults collected from pots were placed as a group (five thrips) on onion plants
June 1986
& SHELTON: OVERWINTERING
NORTH
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Mean number (from nine sample units) of T. tabaci adults recovered from wheat and alfalfa after an initial inoculation of 25 T. tabaci adults per sample unit, 1982-83, Ontario County, N.Y.
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contained within clear plastic cylinders with Nytex lids. The onion plants were placed in a growth chamber at 27°C, 16:8 (L:D) photoperiod, and were examined 3 weeks later for T. tabaci larvae and adults. This was done so that we might determine if the females that overwintered could produce viable offspring. Suitability of Hosts for T. tabaci. In 1984, winter wheat (Triticum vulgare ViII.), alfalfa (Medicago sativa L.), oat (Avena sativa L.), barley (Hordeum vulgare L.), Kentucky bluegrass (Poa pratensis L.), timothy (Phleum pratense L.), and smooth brome (Bromus inermis Leyss) were tested to ascertain if T. tabaci could oviposit, and whether any eggs would hatch and larvae develop on these plants. Timothy, smooth brome, and Kentucky bluegrass were chosen because they were present in significant proportions in the ditch areas. Barley is used as a winter cover crop, but it is also planted in the spring between the rows of onions to reduce soil erosion of muck soils and, thus, may serve as an initial breeding site or nutritional source for T. tabaci until the onions become suitable for colonization. The seeds of these plants were planted at commercial densities in pots (13 cm diam) and were grown in the greenhouse at 20-27°C, 16:8 photoperiod. After growing for 6 weeks in the greenhouse, six pots of each plant species were selected and taken to a growth chamber set at 27°C, 16:8 photoperiod. This procedure was replicated four times in 1984. A clear plastic cylinder was placed over the plants, five adult female T. tabaci were added, and a Nytex lid was placed on the top of the cylinder. After 5 days, the stems and shoots from three pots of each plant species were cut at the base and the plant material was placed into a beaker with 50% ethanol. When the chlorophyll had dissolved, the stems and shoots were examined microscopically for thrips eggs. After another 7 da ys, the remaining three pots per species were
------~. •.
U)~ Q..
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-0-0
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10
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FOLIAGE FOLIAGE
·.WHEAT SOIL .o-ALFALFA
SO I L
2
0---.0
13
DEC
3
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JAN
FEB
6
27
MAR
Fig. 2. Mean number (from nine sample units) of T. tabaci adults and larvae recovered from wheat and alfalfa after an initial inoculation of 25 T. tabaci per sample unit during 1983-84, Ontario County, N.Y.
examined for larvae. The shoots and stems were carefully cut at the base and placed into 70% ethanol. The plant material was removed after 5-10 min, the solution strained through a Nytex screen, and the larvae counted. Results and Discussion Survey of Overwintering Thrips. Six species of adult thrips were collected during the winter of 1982-83: T. tabaci, Anaphothrips obscurus (MulIer), Chirothrips manicatus Haliday, Frankliniella tritici (Fitch), Frankliniella tenuicornis (Uzel), and Aptinothrips rufus (Gmelin), as well as larvae from the tribes Thripini, Anaphothripini, and Chirothripini (Table 1). With the inclusion of barley in 1983-84, Limothrips denticornis (Haliday) adults were also collected. The mean number of thrips per sample was low throughout both winters, and this is consistent with the findings of Shirck (1951) and Boyce & Miller (1953) of
